Smelting of zinc in ceramic retorts



3,fl33,fl74 Patented May 8, 1962 3,033,674 SMELTING OF ZlNC IN CERAMXCRETURTS Kenneth A. Phillips, Collinsville, llh, assignor to AmericanZinc, Lead and Elmelting Company, St. Louis, Mo., a corporation of MaineNo Drawing. Filed Dec. 16, 1959, Ser. No. 859,862 ll tClaim. (Cl. 7586)This invention relates to the smelting of zinc. One process by whichzinc metal is commonly produced involves mixing zinc oxide, which may bein the form of Zinc sinter, with carbonaceous reducing material andsodium chloride, charging banks of horizontal retorts with the mixture,heating the retorts externally, and condensing the reduced zinc metal.The retorts are commonly hollow cylinders of ceramic material, orientedtransversely of a long furnace, with their inner ends supported on alongitudinal center wall and with their mouths opening through a sidewall of the furnace, much like boiler tubes in a header. The retorts arearranged in banks symmetrically about the center wall, so that twochambers full of retorts are provided, one on either side of the centerwall, the retorts opening through opposite long side walls of thefurnace. The retorts are heated, externally, by hot gases in the spacebetween the center wall and the side wall through which the retortsopen. The retorts, which are about ten inches in (outside) diameter, andfive feet long, must be substantially 'imper vious to Zinc vapor,resistant to attack by coal ash and the gangue constituents of zinc ore,must have high strength and good resistance to plastic flow attemperatures up to about 2300 F. They must be transferred, at high heat,from the firing kiln or a soaking pit, to the furnace; a retort whosetemperature after firing drops below 1600 F. being almost certain tofail quickly. Industry wide, the average life of silica retorts is about45 days. It can be seen, then, that it is of the utmost importance thatthe deterioration of the retorts, which is rapid enough as it is, be notaccelerated.

Immediately outside the side walls of the furnace, the open ends of theretorts are capped with condensers, which are in the form of truncatedcones. The ends of these cones are stuffed with wet anthracite or coke,in such a way that the carbon monoxide produced in the reaction betweenthe carbonaceous material and the Zinc oxide, can escape, and that mostof the metal fumes cannot. The condensers are maintained at a heatwillciently high to keep the Zinc molten, but low enough to condense themetal vapors. For example, the retorts may be kept at about 2150 F. andthe condensers at about 950 F. From time to time, the plug in thecondensers is knocked out, and the zinc metal is tapped.

As has been indicated above, sodium chloride, common salt, is used inthe charging mixture. Without the use of this salt or other suitablechloride, the recovery of zinc metal is relatively low, because of theformation of what is known as blue powder, fine particles of zinc metalcoated with a substantially monomolecular layer of zinc oxide. Thismetal is not lost in the process, but simply recirculates, lowering theefliciency of the process. When sodium chloride or zinc chloride isused, the condenser surface is coated with a flux, which encourages thewetting of the surface by the zinc metal, and permits the recovery in asingle cycle of about 85% of the zinc vapor as molten zinc. In ahigh-lead zinc ore, the presence of the lead facilitates the condensingof the zinc. When a low-lead zinc is desired, a zinc ore low in leadmust be used, and the recovery of the zinc in liquid form is morediflicult, and the use of an efiicient halide flux is doubly important.

It was suggested in an early patent (Moulden et al., No. 1,030,676) thatother halides might be used as fluxes in the reduction process, butchlorides are the only specific examples given. It has been known insoldering fluxes, to use a mixture of chlorides and fluorides. It hasalso been known to use a mixture of chlorides and fluorides in therecovery of zinc metal from dross, cf. Lytle, No. 2,578,977 andDeterding, No. 2,701,194. The addition of fluorides has been known totend to make the contaminated metal easier to coalesce into a liquidpool. However, in the primary smelting process, the use of even onepercent of fluorite by weight of sinter results in a very serious attackon the refractory walls of the retorts. The degree of attack is roughlyproportional to the percentage of fluorite added. Although the evidenceis somewhat difficult to evaluate, it is believed that the degree ofattack is also increased when coarse particles of fluorite are employed.

One of the objects of this invention is to provide a more efficientmethod of smelting zinc ores in ceramic retorts, which utilizes thefluxing capacity of fluorides without seriously damaging the retorts.

Other objects will become apparent to those skilled in the art in thelight of the following description.

In accordance with this invention, generally stated, a process ofsmelting zinc ores in refractory retorts is provided, in which arelatively small amount of finelyground fluorite is used in conjunctionwith sodium chloride, in such a way as to unexpectedly increase therecovery of metallic Zinc over the recovery to be expected from the useof either the fluorite or the salt, singly. The process of thisinvention does not lead to uneconomic deterioration of the retorts.

The fineness to which the fluorite is ground seems to determine itseflectiveness as a fluxing medium, its efliciency varying, withinlimits, inversely as the particle size. However, merely usingfinely-ground fluorite alone does not give the desired results. In orderto achieve the desired fluxing, it is necessary that the fluorite beadmixed with common salt in the charge mix.

An example of the process of this invention, and a comparison of theresults obtained with it, and the results obtained by conventionalmethods, and results obtained with the use of fluorite alone, is asfollows:

Example 1 Retorts approximately 9% inches in diameter and 61 incheslong, inside dimensions, were filled with thoroughly mixed chargeshaving the compositions shown below.

The charge of retort 1 represents a conventional charge. The charge ofretort 2. represents a simple substitution of fluoride for common salt.In retort 2, the fluorite was ground moderately fine, with of it passinga 200 mesh screen. Retort 3 represents the charge used in the process ofthis invention. The fluorite in retort 3 was ground sufi'iciently fineso that all of it passed a 200 mesh screen.

The temperature ofthe gases surrounding the retorts at the beginning ofthe forty-eight hour cycle was 1750 F.; the temperature at theconclusion of the cycle was 2220 F. The zinc metal was drawn four timesduring the'cycle.

and filtered to recover the solid contents thereof. The

following data represent the averages for a number of retorts chargedand treated in the same way.

The ladle sk-immings are drawn from the condenser, along with the liquidmetal therein. They contain the coal or coke used to stuff the condenserand all zinc particles or zinc compounds which are not collected asliquid metal and cast into slabs. The ladle skimmings and rich residuesfrom the front of the retort are recirculated in the next mixed charge.The lean residues are discarded or are treated by some method other thanretort smelting. In commercial operations the condenser fume is notcollected; it is wasted to the atmosphere.

The simultaneous use in retort 3 of 1.67% of salt and 0.20% of fluorite,based upon the sinter weight, raised the output of slab zinc by over 3%.In retorts 1 and 2 the use of one halide alone allowed an equivalentamount of zinc to partially oxidize and to enter the blue powder. Thelatter is recirculated on the following charge, and most of the zinc isrecovered in the second pass through the furnace, 'but the retreatmentstep reduces furnace capacity and increases operating costs.

It should be noted that the large amount of fiuorspar in the charge ofretort 2 had a seriously deleterious effect upon the ceramic surface ofthe retort, whereas the small amount of fine fluorspar of the charge inretort 3 had no discernible deleterious eflect upon the retort.

Example 2 Retorts approximately 9% inches in diameter and 61 incheslong, inside dimensions, were filled with thoroughly mixed chargeshaving the compositions shown below. The fluorite in retort 1 was groundvery finely, so that all of it would pass a 200 mesh screen. Thefluorite in retort 2 was ground only to pass through a 10 mesh screen.

Dry pounds of material charge Retort 1 Retort 2 Sinter, 65% Zn 180 180Reduction fuel, 86% C 55 55 Salt, NaCl 3 3 Fluorite, GaFz .36 1.08

The temperature of the gases surrounding the retorts at the beginning ofthe 48-hour cycle was 1750 F.; the

temperature at the conclusion of the cycle was 2220" F. The zinc metalproduced was drawn four times during the The table above shows that 0.36lb. of fine fluoride was as eiiective in promoting the condensation ofzinc as was 1.08 lbs. of coarse fluorite. To be eifective, the fluorinein the fluorite must be carried to the condenser, where it will enter achloride-fluoride flux. The following table shows that substantiallyequal amounts of fluorine travel to the condenser when a small amount offine fluorite is used as when a larger amount of coarse fluorite isused.

Assay percent fluorine in products Retort 1 Retort 2 Trial 1 Trial 2Trial 1 Trial 2 Ladle skimmings 25 28 24 26 Condenser fume:

Before 1st draw 18 13 20 .15

1st to 2nd draw .08' .09 .09 .19

2nd to 3rd dra .11 17 14 .14

3rd to 4th draw 15 22 20 .13 Averages l3 15 16 15 The metallurgicalresults were therefore alike; the physical results are diiferent. Thesmall amount of fluorite used in the retorts marked 1 did not seriouslydamage the retorts used; the larger amount of coarse fluorite usedattacked the retorts marked 2 seriously enough to render the use offluorite uneconomic at some smelters.

Example 3 Retorts approximately 9% inches in diameter and 61 incheslong, inside. dimensions, were filled with thoroughly mixed chargeshaving the compositions shown below. The fluorite in retort 1 was groundvery finely, so that all of it would pass a 200 mesh screen. Thefluorite in retort 2 was ground only through a 10 mesh screen.

Dry Pounds of Material Charged Retort 1 Retort 2 Sinterchargedmoocoo-(7::

Sinter, 65% On Reduction fuel, 86% 0-- Salt, NaCl Fluorite, CEtFz andtreated in the same way.

Pounds of zinc Dry pounds of contents product Retort 1 Retort 2 Retort 2Slab zinc drawn Rich residues- Lean residues-.- Condenser fume 5 5 Thisseries of tests showed that the beneficial efiects of fluoritesubstantially disappeared when the amount 0t fine fluorite used wasdecreased below 0.1% of the smter weight or when the amount of coarsefluorite used was a 6 pie 1. No significant attack of the fluorite wasnoted and converted to zinc vapor, and condensing said zinc upon theretorts used in the series of tests covered by vapor. Example 3.

It is therefore believed that the amount of fluorite added should not beless than 0.1% nor more than 0.6% of 5 References (med m the file ofthis patent the sinter Weight. The best balance between recovery UNITEDSTATES PATENTS and retort life is achieved when 0.2% to 0.3% of finely-79,701 Stevens July 7, 1868 ground fluorite is used, with about 0.5 to2.0% of com-1 713,043 Armstrong Nov. 4, 1902 mon salt, by weight of thesinter. At least 90% -of the 1,999,209 Queneau Apr. 30, 1935 fluoriteshould pass a 150 mesh screen. 10 2,578,977 Lytle Dec. 18, 1951 Havingthus described the invention, what is claimed 2,701,194 Deterding Feb.1, 1955 and desired to be secured by Letters Patent is: 2,867,037 LawtonJ an. 6, 1959 The method of smelting Z1110 ore 1n ceramic retorts IGTHER REFERENCES comprising charging said retorts with a mixture ofoxidic zinc ore, reduction fuel, sodium chloride and fluorite, at HackhsChemical Dictionary, by Julius Grant, 3rd least of the fluorite being ofa fineness at least to Edition, The Blakiston Co., 1944, page 214 reliedupon.

pass a 150 mesh screen, said sodium chloride being pres- ChambersTechnical Dictionary, by C. F; Tweney 'ent in the charging mixture in anamount between about and L. E. C. Hughes, Revised Edition withSupplement,

0.5 and 2.0 percent by weight of the oxidic zinc ore and 1944,theMacmillan Co., page 177 relied upon. the said fluorite being presentin the charging mixture in 20 Chemical Encyclopedia, by C. T. Kingzett,3rd Edition,

an amount between about 0.1 and 0.6 percent by Weight D. Van NostrandCo., 1924, page relied upon.

of the oxidic zinc ore, heating said retorts and said mixture ChemicalEngineers Handbook, by Perry, 3rd Edition, to a temperature at which,over a period of time, the ma- McGraw-Hi1l Book Co. Inc, 1950, page 1111relied jor portion of the oxidic zinc ore is reduced to metallic zincupon.

